The present invention relates to a safety switch.
Safety switches are well known, and are typically used to prevent access to for example electromechanical machinery when that machinery is in operation. In a conventional arrangement the safety switch is mounted on a doorpost of a machinery guard, and an actuator for the safety switch is mounted on a corresponding door. When the door is closed the actuator engages with the safety switch, which in turn closes a set of electrical contacts which allow power to be supplied to the machinery. This arrangement ensures that power can only be supplied to the machinery when the guard door is shut. When the guard door is opened, the actuator disengages from (i.e. is withdrawn from) the safety switch, thereby opening the electrical contacts and cutting off the supply of power to the machinery.
A typical safety switch comprises a body, in which is provided a set of contacts fixed in position relative to the body. An axially slideable plunger is mounted inside the body, and is moveable relative to the body. The plunger (or another plunger in contact with the plunger, for example a contact block plunger) is provided with another set of contacts. The plunger is biased towards a cam arrangement or other control arrangement by a biasing element, such as a spring. The actuator mentioned above is arranged to engage with the cam arrangement. DE102004038488 discloses an example of a safety switch having a contact block plunger that is movable in response to engagement or withdrawal of an actuator to effectuate the making or breaking of the electrical connection of the safety switch.
In many safety switches, if the actuator is not engaged with the cam arrangement (e.g. if the actuator is not engaged with the safety switch), the cam arrangement is arranged to prevent the contacts on the plunger coming into contact with the contacts in the body of the switch by preventing movement of the plunger (i.e. the plunger is kept in a first plunger position). By preventing the contacts from contacting one another, the switch cannot conduct electricity while the actuator is not engaged with the cam arrangement.
Bringing the actuator into engagement with the cam arrangement causes the cam arrangement to rotate, which in turn causes the plunger (which is biased toward the cam arrangement) to move into a notch provided in the cam arrangement. The plunger is then in a second plunger position. When the plunger moves into the notch, the contacts on the plunger are brought into contact with the contacts of the body of the switch, allowing electricity to flow through the safety switch.
In many safety switches, the plunger is provided with two independent bridge contacts, which are moveable (e.g. with movement of the plunger) to each bridge to fixed contacts provided in the body of the safety switch. The use of two bridge contacts and corresponding fixed contacts provides some redundancy and/or added safety functionality. A safety switch, or a configuration of which the safety switch is a part, is frequently configured so that both bridge contacts of the plunger need to be brought into electrical connection with their respective fixed contacts in the body of the safety for the safety switch as a whole to conduct electricity (or, more generally, for the arrangement of which the safety switch is a part to, as a whole, conduct electricity). If damage to the safety switch results in one of the bridge contacts becoming deformed or damaged or the like, or simply not moveable into connection with the respective fixed contacts, the safety switch as a whole, or an arrangement of which the safety switch forms a part, cannot conduct electricity.
The use of two bridge contacts and corresponding fixed contacts improves the safety and functionality of a safety switch. However, the safety and functionality could be further improved. For instance, one or more parts of the safety switch may become damaged or destroyed, and the damage or destruction may result in the bridge contacts of the contact block plunger being brought into and/or kept in contact with the fixed contacts of the body of the safety switch. Thus, even though the safety switch is damaged, the safety switch, or a configuration of which the switch forms a part, may still be in a conductive state. This may be the case even if an actuator is not engaged with the safety switch. In theory at least, this means that a user could enter a machinery guard while the machinery inside is still powered and/or operating, defeating the purpose of the safety switch. It is desirable to avoid this situation.
One solution to the above-mentioned problem is to provide more than one safety switch. If one switch is damaged, the other might still be operational. However, this can lead to cost implications, such as the need to purchase an additional safety switch, and to install and maintain this additional safety switch. Furthermore, in some situations it may be difficult to install a further safety switch due to spatial limitations or the like.
It is therefore an object of the present invention to provide an improved or alternative safety switch or safety switch assembly which may overcome or substantially mitigate at least one disadvantage of the prior art, whether identified herein or elsewhere.